A pneumatic tire is molded and vulcanized in a mold generally having two opposed sections for shaping the tire. Each section has an annular ring or matrix which is separately produced, for example, by casting. The tread rings each have a geometric pattern of configured ridges extending from the normally smooth inner periphery of the ring, for molding in the tire tread a correspondingly configured pattern of slots which improve the traction characteristics of the tire, especially on rain-covered roadways where the slots act as channels through which the water can escape from the tread as it contacts the roadway.
It is desirable to cast the ridges with the tread ring. Many times this is not possible because of the small size of some of the slots. Ridges cast too small are susceptible to breakage which is detrimental to the mold. Therefore, such ridges are separately machined from material which is preferably stronger and more durable than material used in casting the tread ring. The small separate ridges fastened to the mold are called blades or blading, and shape the smaller slots or recesses generally molded in the tread ribs projecting from between the larger slots or grooves molded in the tread by the ridges or projections integrally cast with the tread ring.
Most blades have an even thickness throughout their length and provide a correspondingly configured recess having a uniform width. For example, U.S. Pat. No. 3,553,790 shows and describes a typical blade which may be provided with air vents or passageways for allowing the escapement of gases during the vulcanization of a tire. However, even in this case, the blades are designed to produce configured recesses having a uniform width, since the V-shaped grooves used as air vents are purposely made sufficiently small to prevent rubber from flowing into the air vents and clogging them during the vulcanization process.
A pair of sharp edges are formed on the opposing sides of each recess and grip the roadway to increase the traction characteristics of the tire. However, these uniformly shaped recesses are susceptible to closing as they move into contact with the roadway because of the deflection or flexing characteristics of the tire as it engages the roadway. It is desirable that these recesses remain at least partially open to prevent a serious reduction of the traction characteristics of the tire. The invention is directed to providing a highly improved blade for molding a recess having a non-uniform cross-sectional width throughout its length, thereby increasing the probability that at least a portion of the recess will remain open as the tire contacts the roadway.
Briefly stated, the invention is in a mold in which tires are molded and vulcanized. The mold comprises a tread ring including configured projections which are integrally cast in, and extend from, the ring for forming correspondingly configured grooves in the tread of a tire placed in the mold and vulcanized. A plurality of blades are separately shaped apart from the ring and fastened to the ring during the process for forming the finished ring, including the projections and blades. Each of the blades has a varying thickness and a configuration and size sufficient to produce a correspondingly configured recess in the tread of a vulcanized tire, such that a portion of the recess having a larger width will be at least partially open when a portion of the recess having a smaller width, closes.